The present invention is directed to apparatus and a method for automatically measuring the blood pressure of an individual and specifically to apparatus and a method for inflating a pressurized cuff to reliably achieve a predetermined initial pressure level.
A conventional automatic blood pressure gauge includes a resilient inflatable cuff and an electric pump. The pump is controlled by a microprocessor to inflate the cuff with a fluid, such as air, to a preset pressure. In addition, this automatic gauge includes a pressure transducer that measures the instantaneous air pressure levels in the cuff. The pressure signal produced by the transducer is used to determine both the instantaneous air pressure of the cuff and the blood pressure pulse of the individual. This pressure signal is generally digitized and processed by the microprocessor to produce values representing the systolic and diastolic blood pressure measurements of the individual.
In operation, the cuff is affixed to the upper arm area of the patient and is then inflated to a pressure greater than the suspected systolic pressure, for example, 150 to 200 millimeters of mercury (mmHg). This pressure level collapses the main artery in the arm, effectively stopping any blood flow to the lower arm. Next, the cuff is deflated slowly and the transducer pressure signal is monitored to detect variations in cuff pressure caused by the patient's pulse, which is coupled into the cuff. By monitoring the amplitude of the measured pulse signal, the system can determine the patient's systolic and diastolic pressures.
One exemplary system is described in U.S. Pat. No. 4,949,710 entitled METHOD OF ARTIFACT REJECTION FOR NONINVASIVE BLOOD-PRESSURE MEASUREMENT BY PREDICTION AND ADJUSTMENT OF BLOOD-PRESSURE DATA, which is hereby incorporated by reference for its teaching on automatic blood pressure gauges. This system monitors the patient's blood pressure signal to determine the maximum detected pulse amplitude. This is commonly referred to as the mean arterial pressure (MAP). The systolic and diastolic blood pressure levels are then determined as the respective pressures corresponding to the amplitude of the pulse signal being 60% of the maximum value, prior to reaching the maximum value; and 80% of the maximum value, after reaching the maximum value.
To be most effective, an automatic blood pressure gauge should quickly inflate the cuff to a preset pressure value and then deflate the cuff according to a known deflation curve. It is desirable to complete this task in a relatively short time period, so as to provide quick results and to minimize patient anxiety and discomfort. This task is complicated by differences in blood pressure from person to person and in one person within a single day. For example, the systolic blood pressure of an individual may range between 90 mmHg and 180 mmHg in a single day between periods of sleep and periods of exercise.
Another factor to be considered is the adaptability of the blood pressure gauge. To be as useful as possible, it is desirable for the gauge to operate well with a number of different types of cuffs. These include neonatal cuffs for infants and finger, arm and thigh cuffs for children and adults. Each of these types of cuff may hold a different amount of fluid when inflated to a given pressure. To be generally useful, it is desirable for the blood pressure gauge to inflate each type of cuff to the preset pressure within a predetermined time interval with approximately the same level of accuracy. It is also important that the gauge not overinflate any of the cuffs, especially the small pediatric and neonatal cuffs.
It is desirable to inflate an adult cuff to its initial pressure in five to six seconds. The fluid flow which produces this inflation rate in a large cuff may be too great for very small cuffs, such as those used for neonates. If this flow rate were used for all cuffs, the combined effects of the delay in measuring and in responding to the threshold pressure level, and the inertia in the pump and pump motor may combine to cause pressures much greater than the threshold pressure to be applied to a neonatal cuff.
U.S. Pat. No. 4,493,326 entitled AUTOMATIC BLOOD ? 0 PRESSURE SYSTEM WITH SERVO CONTROLLED INFLATION AND DEFLATION, which is hereby incorporated by reference, operates the pump motor under closed-loop servo control. Signals generated by a pressure gauge coupled to the cuff are used to control the pump motor to maintain a constant inflation rate. The pump is turned off when the level of the pressure signal exceeds a predetermined threshold. The time required to inflate a cuff with this system depends on the volume of the cuff and the selected inflation rate. If the rate is too fast, it is possible that a small cuff, for example, a neonatal cuff, may be over inflated before the system can stop the pump. If the rate is too slow, it may take a relatively long time to inflate a large cuff, such as a thigh cuff, to the desired threshold pressure.
U.S Pat. No. 4,360,029 entitled AUTOMATIC MEAN BLOOD PRESSURE READING DEVICE, which is hereby incorporated by reference, inflates the cuff to a pressure which is 60 mmHg greater than the previously measured mean arterial pressure (MAP). The MAP is the pressure at which the blood pressure pulse signal sensed by the blood pressure gauge has its greatest value. If no value for MAP has previously been determined, the gauge stops the pump when a pressure of 160 mmHg has been reached.